1. Field of the Invention
This invention relates to a process of making separators for an electrochemical cell. The principal object of this invention is to provide a process of making separators for an electrochemical cell of high quality, especially those having relatively uniform micropores and very low electrical resistance in electrolyte.
2. Description of the Prior Art
In general, the qualities requisite to a separator for electrochemical cell are that the separator inhibits any short-circuiting by a contact between electrodes having opposite polarities and yet permits electrode reaction-participating ions to pass freely; that the separator prevents minute particles, such as those of released electrode materials except ions, from migrating through; that the performance of the separator is not lowered by the electrode reactions, nor by chemicals employed in the cell; and that the separator gives no materials harmful to the cell.
Some of the prior separators for lead storage cell are made of wood. Some are produced by treating porous-made paper with a synthetic resin. Some are made by compounding silica gel into rubber. Some are produced by a process in which calcium carbonate powder is compounded into rubber and then the resultant compound is treated with hydrochloric acid to dissolve calcium carbonate. In addition, some of the separators of the prior art are made by sintering powdery polyvinyl chloride.
In the case of prior separators for a cell employing an alkaline electrolyte, nylon fabrics, cellophane etc. are employed.
Among these separators, those made of wood or by sintering powdery polyvinyl chloride have lower limits of thickness at a relatively high level and are not suitable where the space factor is of importance. Separators which have been made by adding a powdery inorganic material to rubber, by sintering polyvinyl chloride powder, or by employing nylon fabric have micropores having sizes varying over a wide range and are not satisfactory for preventing the migration of released electrode materials, even when the mean pore radius is controlled to be a lower value.
Separators produced by treating porous-made paper with a synthetic resin are not satisfactory with regard to their microporosity, and their mechanical strength is lowered due to sulfuric acid when employed in a lead storage cell. Separators made of polyvinyl chloride can generate chloride ions under the influence of electrode reactions, and electrode materials of a lead storage cell may lose weight.
Further, separators made from cellophane have low mechanical strengths. In short, the various separators of the prior art have several disadvantages along with several advantages, and are not satisfactory.
Generally speaking, both pore sizes and the electric resistance of a separator impregnated with electrolyte, which constitute the elementary characteristics of a separator for an electrochemical cell, depend upon the process of producing or making the separator. To select the starting material is important, especially when it is intended to make a separator which does not easily lose weight by electrolyte and electrode reactions and scarcely generates harmful materials. The mechanical strength and the thickness of the separator are determined by the kinds and the combination of the starting materials.